This invention is about the techniques used in producing compositional coatings, when the coating is formed by spraying molten materials onto a substrate and when such coatings are employed in various equipment manufacturing, energetics, metallurgical and other areas to protect various details and products from corrosion, gas erosion and heat and to impart new properties to these parts. Such compositional coatings can be used as a substrate for application of other coatings, such as polymer coatings, or as a substrate for saturation with various mixtures, including catalytic compounds.
Techniques of applying protective coatings over metallic or ceramic surfaces by thermal gas spraying, employing compositions consisting of aluminum compounds are known. The most widely known technique to form compositional coatings is the use of aluminum powder consisting of aluminum oxide (see "Metallic and Ceramic Coatings: Production, High Temperature Properties and Application", M. G. Hocking, V. Vasantasree and P. S.Sidky, Materials Department, Imperial College, 1990, London.). As a rule, gaseous thermal spraying of ceramic coatings is carried out over previously deposited metallic precoat.
A deposition of aluminum precoat, which assures high anodic characteristics and a high resistance to erosion and corrosion is known (U.S. Pat. No. 4,238,233, TKP C 22 C21/10, 1981). An Al-Zn-Mg coating with indium, bismuth and tin additives is deposited on the inner surfaces of pipes and assures their cathodic protection.
A technique forming wear resistant coatings is known (see SSSR Author Certificate No. 2026890, TKP C 25 D, 1992), which includes the deposition of the main coating based on aluminum and containing Al,Cu,Mg,Mn (close to the composition of the alloy D 16), which is then oxidized by microarc in an alkaline electrolyte over a precoat of an alloyable system of Zn-Cu-Al-Ni-B alloy. This technique allows to improve the coating's adhesion strength to the substrate and simultaneously increases its wear resistance.
A plasma spraying process of ceramic coatings is known (see SSSR Author Certificate No. 2021389, TKP C 25 D 11/ 02, 1994) in which a plasma spray deposits a precoat (elastic nickel alloy with chromium and aluminum-nichrome) of a 0.2 mm thickness onto a metallic substrate and over this precoat a ceramic coating is deposited, employing various powder blends: partially stabilized zirconium dioxide, aluminum oxide, titanium oxide, chromium oxide and by heating such blends to a temperature of 150-200.degree. C., onto a substrate surface which has been preheated to a temperature of up to 1300.degree. C. In addition a coating of ceramic particles at 20-80.degree. C. is sprayed over the precoat. This technique allows to substantially improve the thermal fatigue of the product.
The techniques mentioned above are complex, requiring a high amount of labor and a number of additional labor consuming operations: precoat deposition, oxidation and continuous temperature control of the substrate and the coating.
A technique in obtaining a multilayer coating is known (see SSSR Author Certificate No. 2049827, TPK C 23 C 4/00, 1995), encompassing spraying of the coating in the inert gas and disassociated hydrogen atmosphere. In this case, a precoat of Al-Ni powder is sprayed, which in the presence of inert gases and hydrogen ions forms hydrated aluminum oxide structures. The main coating is applied employing such powder, or the blends based on aluminum or nickel and chemically inert additives with a laminar structure, such as aluminum nitride or carbon. The coating obtained in this manner has in its composition aluminum, hydrated aluminum oxide species and boron nitride or carbon additives. The latter act as solid lubricants and assure the resistance to wear. A polymeric coating is applied over the coating prepared in this manner. Hydrated aluminum oxide types formed during the spraying improve the corrosion resistance.
It must be pointed out, however, that the coating obtained by the technology described above, is quite expensive, because of the large number of intermediate operations and expensive materials used in spraying, for example aluminum-nickel powder. In addition, on the basis of the above technology, such coating cannot have a sufficiently high resistance to wear and a sufficiently high porosity, which makes it unsuitable as a substrate for later impregnation with various mixtures i.e. the coating is not universally useful.
The method in obtaining a catalytic carrier based on intermediate aluminum hydroxide phases and {character pullout}-phase is known, where the chemical reaction is carried out on a ceramic matrix surface in aluminate, silicate and sodium sulfate solutions at pH 10.5-11.5, followed by later reduction of silicon containing aluminum hydroxide from the solution (European Patent No. EP 537871, 1994).
The main feature of this method is the high uniformity level of the deposited coating, the main disadvantage is that a reliable adhesion level between the catalyst carrier and the ceramic substrate is not obtained.
A method to deposit a catalyst carrier to steel or aluminum sheets is known (Japanese Patent No. 56-078450, 1981). The technique includes spraying at high temperature of an aqueous suspension containing glass slag, metal oxide catalyst, and also silicon oxide and/or aluminum oxide.
It is clear that introduction of glass slag into the spraying composition is to develop a free surface and to form a specific macrostructure, which together with the microstructure of the catalytic coating, must assure the optimum combination of the catalytic properties at changing conditions. However, as demonstrated in practice, the adhesion strength of such compositions obtained by spraying aqueous suspension to the substrate is not high and does not assure a reliable performance at adverse conditions.
A method is known for depositing plasma coatings (see SSSR Author Certificate No. 1528810, TPK C 23 C 4/04/1989) by employing powder mixtures: aluminum oxide and metallic titanium (1 5-60%) as a metallic binder. This method allows to obtain coatings resistant to abrasive wear. The porosity of such coating is 8-11 %, cohesive strength 9-10 Mpa. However, if such coating is to be universally useful aced for many applications, including as a precoat for other coatings, the problem of its adhesion strength arises: from one side with the substrate and from the other side with the coating which is subsequently applied to the surface of this precoat. As indicated by experimental work in this area, the solution of this problem requires that intermediate layer (precoat) must have, firstly, a thermal expansion coefficient (t.e.c.) as close as possible to the t.e.c. of the substrate and, secondly, must have a maximum integration into the structure i.e. the atoms of the coating material must form metallic bonds with the atoms of the substrate material. If the first condition can be easily met by selection of the material for the plastic bond, a metal of t.e.c. close to that of the substrate material (titanium in this case), the condition of a high degree of integration is fulfilled only under condition that alloy type of compounds are formed at the interface between the precoat and the substrate. During the plasma spraying, similar conditions can be realized by carrying out a high temperature treatment after spraying; in addition the temperature interval on heating must assure an effective bidirectional diffusion of the atoms from the precoat and from the substrate. Since such temperatures are high (above 1000.degree. C.), usually a noticeable degradation of such coating is observed, which is characterized by anomalous growth of grains, high liquidization and considerable oxidation at its edges. Such reheating is not anticipated in the invention under discussion, and even if it were anticipated, the previously mentioned detrimental results would come up.
In order to assure a good adhesion of the precoat with the surface coating being deposited over it, the precoat must have a well developed surface, the relative free surface in this case should be no less than 10 sq.m/g (that means that 1 gram of the coating must have 10 sq.m. total surface), which assures a good adhesion of the sprayed coating. However, by calculating the relative free surface from the claimed porosity (about 10%), it is shown that it does not exceed 1 sq.m/g, which is not sufficient.
Therefore, by using this invention it is not possible to obtain a coating which assures a high adhesion to the substrate and simultaneously a high adhesion with the coatings deposited on its surface.